Backlights can be considered to fall into one of two categories depending on where the light sources are positioned relative to the output area of the backlight, where the backlight “output area” corresponds to the viewable area or region of the display device. The “output area” of a backlight is sometimes referred to herein as an “output region” or “output surface” to distinguish between the region or surface itself and the area (the numerical quantity having units of square meters, square millimeters, square inches, or the like) of that region or surface.
The first category is “edge-lit.” In an edge-lit backlight, one or more light sources are disposed—from a plan-view perspective—along an outer border or periphery of the backlight construction, generally outside the area or zone corresponding to the output area. Often, the light source(s) are shielded from view by a frame or bezel that borders the output area of the backlight. The light source(s) typically emit light into a component referred to as a “light guide,” particularly in cases where a very thin profile backlight is desired, as in laptop computer displays. The light guide is a clear, solid, and relatively thin plate whose length and width dimensions are on the order of the backlight output area. The light guide uses total internal reflection (TIR) to transport or guide light from the edge-mounted lamps across the entire length or width of the light guide to the opposite edge of the backlight, and a non-uniform pattern of localized extraction structures is provided on a surface of the light guide to redirect some of this guided light out of the light guide toward the output area of the backlight. Such backlights typically also include light management films, such as a reflective material disposed behind or below the light guide, and a reflective polarizing film and prismatic BEF film(s) disposed in front of or above the light guide, to increase on-axis brightness.
In the view of Applicants, drawbacks or limitations of existing edge-lit backlights include the following: the relatively large mass or weight associated with the light guide, particularly for larger backlight sizes; the need to use components that are non-interchangeable from one backlight to another, since light guides must be injection molded or otherwise fabricated for a specific backlight size and for a specific source configuration; the need to use components that require substantial spatial non-uniformities from one position in the backlight to another, as with existing extraction structure patterns; and, as backlight sizes increase, increased difficulty in providing adequate illumination due to limited space or “real estate” along the edge of the display, since the ratio of the perimeter to the area of a rectangle decreases linearly (1/L) with the characteristic in-plane dimension L (e.g., length, or width, or diagonal measure of the output region of the backlight, for a given aspect ratio rectangle).
The second category of backlight is “direct-lit.” In a direct-lit backlight, one or more light sources are disposed—from a plan-view perspective—substantially within the area or zone corresponding to the output area, normally in a regular array or pattern within the zone. Alternatively, one can say that the light source(s) in a direct-lit backlight are disposed directly behind the output area of the backlight. A strongly diffusing plate is typically mounted above the light sources to spread light over the output area. Again, light management films, such as a reflective polarizer film, and prismatic BEF film(s), can also be placed above the diffuser plate for improved on-axis brightness and efficiency.
In the view of Applicants, drawbacks or limitations of existing direct-lit backlights include the following: inefficiencies associated with the strongly diffusing plate; in the case of LED sources, the need for large numbers of such sources for adequate uniformity and brightness, with associated high component cost and heat generation; and limitations on achievable thinness of the backlight beyond which light sources produce non-uniform and undesirable “punchthrough,” wherein a bright spot appears in the output area above each source.
In some cases, a direct-lit backlight may also include one or some light sources at the periphery of the backlight, or an edge-lit backlight may include one or some light sources directly behind the output area. In such cases, the backlight is considered “direct-lit” if most of the light originates from directly behind the output area of the backlight, and “edge-lit” if most of the light originates from the periphery of the output area of the backlight.
Backlights of one type or another are usually used with liquid crystal (LC)-based displays. Liquid crystal display (LCD) panels, because of their method of operation, utilize only one polarization state of light, and hence for LCD applications it may be important to know the backlight's brightness and uniformity for light of the correct or useable polarization state, rather than simply the brightness and uniformity of light that may be unpolarized. In that regard, with all other factors being equal, a backlight that emits light predominantly or exclusively in the useable polarization state is more efficient in an LCD application than a backlight that emits unpolarized light. Nevertheless, backlights that emit light that is not exclusively in the useable polarization state, even to the extent of emitting randomly polarized light, are still fully useable in LCD applications, since the non-useable polarization state can be eliminated by an absorbing polarizer provided between the LCD panel and the backlight.